In today's industries there are frequent needs for in-line measurements of a liquid concentration within a substance, such as the content of water, oil, dye, paint, fuel, glue, etc. within materials such as wood pieces, logs, lumber, veneer, paper, powders, cardboard, fabrics, sand, soil, plants, bricks, concrete and the like; or within other liquids such as in the case of oil spills in water. The conventional method of liquid content measurement, such as a content of water, is based on the difference between the weight of the substance before and after heating it at a temperature close to or above the boiling point of the particular liquid. However, such a method requires long periods of time which may be as much as twenty-four hours or longer, and is limited mostly to the measurement of stationary substances. Therefore this conventional method is not practical for in-line applications where virtually instantaneous measurements are required.
Another method of liquid content determination which is popularly used in the industry, is based on spectral selective optical absorption which can be measured in scattered and/or transmitted light beams. Sets of light sources with precisely selected wavelengths are used in this method. One set of wavelengths which is relatively unaffected by the characteristic being measured, is used as a reference. The other set of wavelengths is tuned to the liquid absorption peak level. Several versions of this approach have been disclosed, in which conventional infrared optical sources are applied, as for example described in U.S. Pat. No. 4,266,878, issued to Auer on May 12, 1981; U.S. Pat. No. 4,634,856, issued to Kirkham on Jan. 6, 1987; U.S. Pat. No. 4,788,853, issued to Bell on Dec. 6, 1988; U.S. Pat. No. 3,851,175, issued to Dahlin et al. on Nov. 26, 1974; U.S. Pat. No. 3,965,356, issued to Howarth on Jun. 22, 1976; U.S. Pat. No. 4,052,615, issued to Cho on Oct. 4, 1977; U.S. Pat. No. 4,300,049, issued to Sturm on Nov. 10, 1981; U.S. Pat. No. 4,465,929, issued to Edgar on Aug. 14, 1984; U.S. Pat. No. 4,577,104, issued to Sturm on Mar. 18, 1986; U.S. Pat. No. 4,733,078, issued to Sturm on Mar. 22, 1988; U.S. Pat. No. 4,823,008, issued to Sturm on Apr. 18, 1989; U.S. Pat. No. 4,840,706, issued to Campbell on Jun. 20, 1989; U.S. Pat. No. 4,879,471, issued to Dahlquist on Nov. 7, 1989; U.S. Pat. No. 4,928,013, issued to Howarth et al. on May 22, 1990; and U.S. Pat. No. 5,870,926, issued to Saito on Feb. 16, 1999. It is also known that a sub-millimeter laser instead of the conventional infrared optical sources can be used in this method, as disclosed in U.S. Pat. No. 4,755,678, issued to Izatt et al. on Jul. 5, 1988. The applications disclosed in these prior art patents generally apply to moisture measurements either in soil or paper products.
It is also known in the industry that moisture measurements can be based on absorption of fast neutron rays gamma radiation, examples of which are disclosed in U.S. Pat. No. 4,766,319, issued to Regimand on Aug. 23, 1988 and U.S. Pat. No. 3,748,473, issued to Chen on Jul. 24, 1973. In-line moisture measurements can also be conducted by using NMR sensors. Apparatus for in-line moisture measurements based on a NMR sensor is disclosed in U.S. Pat. No. 5,594,340, issued to Coyle et al. on Jan. 14, 1997.
Meters for measuring electrical properties of a substance are used for liquid content measurements where a relationship between the electrical properties and the liquid of the substance can be established. For example, the moisture content of a substance may be estimated from electrical properties such as resistance, capacitance and conductivity. However, these types of meters are limited to the lower end of the liquid content. The moisture content range which can be measured by the electrical meters is from 6 percent to 27 percent according to The Standard Test Methods (ASTM: D 4444-84). Therefore, the meters cannot be used efficiently outside of the above mentioned moisture content range because high moisture levels cause no substantial change in electrical properties. Examples of moisture measurements based on electrical properties of a material are disclosed in U.S. Pat. No. 4,853,614, issued to Carver on Aug. 1, 1989; U.S. Pat. No. 4,864,850, issued to Price on Sep. 12, 1989; U.S. Pat. No. 5,668,479, issued to Jackson et al. on Sep. 16, 1997; and U.S. Pat. No. 5,801,537, issued to Siddiqui et al. on Sep. 1, 1998.
It is also well known in the industry to uniformly heat a substance and then to monitor temperature of the heated substance for determination of the moisture content or identification of defects in the substance. U.S. Pat. No. 4,612,802, issued to Clarke et al. on Sep. 23, 1986 discloses a method of non-contact moisture measurements in which an area of one surface of a substance is subjected to a predetermined intensity of heat for a selected period of time. The moisture content is determined by measuring the temperature rise of the heated surface of the substance. U.S. Pat. No. 5,357,112, issued to Steele et al. on Oct. 18, 1984 discloses a method of uniformly heating wood and then submitting the wood to an infrared camera to provide a thermal image of the surface of the wood. The thermal images of the preheated wood are used for identification of knots and/or voids in lumber surfaces. These methods for determination of moisture content require uniform heating of the substance to be measured, which may not be practically applicable under some circumstances. Therefore the application thereof is limited.
U.S. Pat. No. 5,270,546, issued to Jamroz et al. on Dec. 14, 1993 discloses a method and apparatus for non-contact moisture measurements based on infrared radiation absorbed by water present within a substance. In this method the surplus of infrared radiation causes modifications of a substance's surface by changing its microstructure. The density of surface modifications relates to the moisture content of the substance, and an analysis of a portion of the substance's surface can accurately approximate the average moisture content of the substance.
Nevertheless, there is still a need for improved methods and apparatus for non-contact and rapid moisture measurements which can be conveniently used in various applications, particularly in a passive manner without the need to apply infrared radiation or heating.